Portable electric heaters including sheathed elements are known. Typically, such heaters include a fan in a housing which blows air through a coiled sheathed element, which heats the air before the air exits the housing. The housing is usually elongate, with openings at its opposing ends to permit ingress and egress of air.
As is well known in the art, the sheathed element 10 includes a resistive wire 12 positioned inside a sheath 14. Typically, the sheath is usually made of a suitable metal material which is a relatively good conductor of heat, e.g., steel. Also, the wire is electrically insulated from the sheath by an insulator 16. The insulator 16 is any suitable insulating material, e.g., magnesium oxide (MgO).
In the prior art, and as schematically shown in FIG. 1A, the sheathed element is grounded. In general, where the element is readily accessible (i.e., where the element could easily be touched by a user, through inadvertence), the element is required to be grounded. Because the sheath is grounded, at any time when current can flow from the resistive wire to the sheath, there is a voltage across the circuit formed by the resistive wire and the sheath (FIG. 1B). Although many prior art sheathed elements generally perform satisfactorily, arcing or failure of the prior art sheathed element is relatively common, and can have serious consequences. Failure of the sheathed element is generally understood to occur due to three different causes, as follows.
First, failure can occur when the resistive wire touches the sheath. (This situation is schematically illustrated in FIG. 1B.) If this were to happen in manufacturing the product would not pass the hi-pot test on the production line and would be rejected. However, if over time the resistive wire (having been properly positioned when the sheathed element was manufactured) were to creep towards the sheath and ultimately contact it, then an arc would occur. It is thought that this occurs due to materials expanding in use, or due to curved or bent elements.
Second, failure can occur due to too much moisture in the insulation inside the sheath. Moisture is conductive, and when heaters sit in humid conditions moisture can be absorbed into the insulation. Too much moisture can also get into the insulation when a sheath is cracked and in contact with moisture. When this happens, current can pass from the resistive wire to the sheath, potentially causing a failure (i.e., if the current leakage is sufficient).
Failure can also occur due to oxidation of the resistive wire. In this case, the resistive wire oxidizes over time, and a scaling build-up occurs. The scales break away from time to time, causing the wire diameter to become smaller, ultimately resulting in mechanical failure of the resistive wire. With each mechanical failure of the resistive wire, the diameter of the resistive wire decreases. Finally, the wire becomes sufficiently small that a hot spot occurs and the sheathed element fails altogether.
As described in U.S. Pat. No. 4,484,243 (Herbst et al.), arcing between an end of the resistive wire and the sheath can result in “zippering” taking place along the sheath (col. 1, at lines 40-64).
Herbst et al. discloses one prior art solution to the problem. In Herbst et al., a protective circuit arrangement to protect sheathed heating elements is disclosed. The protective circuit interrupts ground fault conditions by decoupling the power line from the heating element. Fusible links are used in the protective circuit. However, the protective circuit disclosed in Herbst et al. is not activated until a failure (i.e., a ground fault) has occurred, which means that the sheathed element must first have been damaged, at least to an extent, before the protective circuit decouples the heating element from the power source.
There is therefore a need for an improved heater apparatus which overcomes or mitigates one or more of the disadvantages of the prior art.